Surgical tourniquet systems are commonly used to stop the flow of arterial blood into a portion of a patient's limb, thus creating a clear, dry surgical field that facilitates the performance of a surgical procedure and improves outcomes. A typical surgical tourniquet system of the prior art includes a tourniquet cuff for encircling a patient's limb at a desired location, a tourniquet instrument, and flexible tubing connecting the cuff to the instrument. In some surgical tourniquet systems of the prior art, the tourniquet cuff includes an inflatable portion, and the inflatable portion of the cuff is connected pneumatically through one or two cuff ports by flexible plastic tubing to a tourniquet instrument that includes a pressure regulator to maintain the pressure in the inflatable portion of the cuff, when applied to a patient's limb at a desired location, near a reference pressure that is above a minimum pressure required to stop arterial blood flow past the cuff during a time period suitably long for the performance of a surgical procedure. Many types of such pneumatic surgical tourniquet systems have been described in the prior art, such as those described by McEwen in U.S. Pat. Nos. 4,469,099, 4,479,494, 5,439,477 and by McEwen and Jameson in U.S. Pat. No. 5,556,415 and No. 5,855,589.
Many studies published in the medical literature have shown that the safest tourniquet pressure is the lowest pressure that will stop the flow of arterial blood past a specific cuff applied to a specific patient for the duration of that patient's surgery. Such studies have shown that higher tourniquet pressures are associated with higher risks of tourniquet-related injuries to the patient. Therefore, when a tourniquet is used in surgery, surgical staff generally try to use the lowest tourniquet pressure that in their judgment is safely possible.
It is well established in the medical literature that the optimal guideline for setting the pressure of a constant-pressure tourniquet is based on “Limb Occlusion Pressure” (LOP). LOP can be defined as the minimum pressure required, at a specific time in a specific tourniquet cuff applied to a specific patient's limb at a specific location, to stop the flow of arterial blood into the limb distal to the cuff. The currently established guideline for setting tourniquet pressure based on LOP is that an additional safety margin of pressure is added to the measured LOP, to account for variations in physiologic characteristics and other changes that may be anticipated to occur normally over the duration of a surgical procedure.
Some surgical tourniquet systems of the prior art include means to measure LOP automatically. Prior-art tourniquet apparatus having automatic LOP measurement means are described by McEwen in U.S. Pat. No. 5,439,477 and by McEwen and Jameson in U.S. Pat. No. 5,556,415. Such prior-art systems have included blood flow transducers that employ a photoplethysmographic principle to sense blood flow in the distal limb, although other transducers have been suggested in the prior art to measure blood flow based on other principles. A blood flow transducer employing the photoplethysmographic principle uses light to indicate the volume of blood present in a transduced region, consisting of a combination of a residual blood volume and a changing blood volume resulting from arterial pulsations. An additional pressure margin based on recommendations in published surgical literature is added to the automatically measured LOP to provide a “Recommended Tourniquet Pressure” (RTP), as a guideline to help the surgical staff select the lowest tourniquet pressure that will safely stop arterial blood flow for the duration of a surgical procedure. Such prior-art systems allow the surgical staff to select the RTP, based on LOP, as the tourniquet pressure for that patient or to select another pressure based on the physician's discretion or the protocol at the institution where the surgery is being performed.
In U.S. Pat. App. No. 20060253150, McEwen and Jameson describe surgical tourniquet apparatus for automatically measuring LOP that overcomes many of the limitations of prior-art apparatus in four principal areas: safety, probability of successful LOP measurement, speed of LOP measurement, and accuracy of LOP measurement. The McEwen '150 apparatus does not introduce secondary hazards associated with the measurement of LOP, has a high probability of successful completion after LOP measurement is initiated, completes LOP measurement sufficiently fast so that the measurement of LOP does not disrupt or unduly delay normal activities in the operating room, and results in an LOP measurement that is accurate within surgically acceptable expectations so that it can be used as the basis for optimal setting of tourniquet pressure prior to inflation of the tourniquet cuff to facilitate surgery.
Despite the improved performance of the McEwen '150 apparatus in measuring LOP, there is one significant limitation: the apparatus does not measure or estimate any changes to LOP that may occur during surgery. Instead, the Recommended Tourniquet Pressure (RTP) equals the sum of the LOP measured prior to cuff inflation for surgery plus a predetermined margin of safety. The margin of safety is set to be greater than the magnitude of any increase in LOP normally expected during surgery and may be dependent on the magnitude of the LOP, as described in McEwen '150. As a result, a constant RTP based on LOP measured prior to surgery may be higher than necessary to the extent that the magnitude of any increase in LOP during surgery is less than the magnitude of the predetermined margin of safety. Additionally, if LOP decreases during surgery below the LOP measured prior to surgery, then the RTP will be unnecessarily high by an even larger amount.
Several variables affecting LOP have been described in the prior art. Prior to surgery, LOP is affected by variables including the patient's limb characteristics (for example, limb shape, circumference and soft tissue characteristics at the cuff location), characteristics of the selected tourniquet cuff (for example, cuff design, cuff shape and cuff width), the technique of application of the cuff to the limb (for example, the degree of snugness or looseness and the absence, presence and type of underlying limb protection sleeve), physiologic characteristics of the patient including blood pressure and limb temperature, and other clinical factors (for example, the extent of any elevation of the limb during LOP measurement and the extent of any limb movement during measurement). After inflation of the tourniquet cuff to facilitate surgery, ongoing LOP during surgery is affected by variables including: the anesthetic technique employed (for example, whether a general or regional anesthetic is given, the types and dosages of anesthetic agents employed and the degree of attention paid to anesthetic management); the length of tourniquet time; isolation of the operative limb from systemic circulation; any change in limb position during surgery; and by any shift in the location of the cuff relative to the limb during surgery. Some but not all of these intraoperative variables change the LOP from the initial level of LOP measured before surgery by changing the patient's blood pressure, one of the variables affecting LOP.
The prior art describes a wide range of tourniquet apparatus for changing tourniquet pressure on the basis of a patient's intraoperative blood pressure. For example, in U.S. Pat. No. 4,479,494 McEwen describes pneumatic tourniquet apparatus in which pressure in a tourniquet cuff may be varied “adaptively” in response to changes in the patient's intraoperative systolic blood pressure. The apparatus of McEwen '494 selectably activates a pressurizing mechanism and a pressure relief mechanism to automatically maintain a substantially constant pressure difference between pressure in the tourniquet cuff and the patient's changing systolic blood pressure during surgery. McEwen '494 includes a tourniquet cuff having one cuff segment for occluding blood flow into the patient's limb and another cuff segment for sensing the patient's systolic blood pressure and includes recorder means for periodically recording the operational status of the apparatus. In addition to McEwen '494, others have described prior-art apparatus for setting or changing tourniquet pressure based on a patient's blood pressure, for example: Lemelson et al in U.S. Pat. No. 4,321,929; Miller et al in U.S. Pat. No. 4,671,290; Ulrich in U.S. Pat. No. 5,569,304; Gruenfeld et al in U.S. Pat. No. 5,842,996; and Hovanes et al in U.S. Pat. No. 6,605,103. However, such prior-art apparatus does not take into account many of the above-described variables that affect limb occlusion pressure and thus the tourniquet pressure is not optimal.
No apparatus known in the prior art adapts tourniquet pressure as a function of changes during surgery in a patient's limb occlusion pressure from an initial limb occlusion pressure. The present invention addresses the need for improved surgical tourniquet apparatus for automatically adapting the pressure applied to a patient's limb by a tourniquet cuff as a function of changes in the patient's limb occlusion pressure during surgery.